Universal Mobile Telecommunications Systems (UMTS) is based on the W-CDMA technology and is in the heart of the 3G communication networks providing packet based transmission of text, digitized voice, and multimedia data at rates up to 2 Megabits per second.
One particular case of UMTS is defined in the 3G specification, where one base station of such a communication systems may comprise two emitting antennas each transmitting one signal comprising one corresponding Common Pilot CHannel (CPICH). The transmitted sequence of pilots from each antenna is constructed by applying a sign pattern given by FIG. 1 to the symbol(1+j), where j is to the imaginary complex such as j2=−1.
FIG. 1: This figure represents the pattern of the transmitted CPICH pilots' sequence.
Those two pilot signals which are transmitted by the base station are used for the determination of the channel characteristics but also for estimating the frequency offset between the base station reference clock and the internal clock of the User Equipment (UE). The received signal corresponding to the transmission of the kth pilot sample in the absence of frequency offset is given by:
      r    ⁡          [      k      ]        =                    (                  1          +          j                )            ⁢              (                                            h              1                        ⁡                          [              k              ]                                +                                                    (                                  -                  1                                )                                            ⌊                                                      k                    +                    1                                    2                                ⌋                                      ⁢                                          h                2                            ⁡                              [                k                ]                                                    )              +          n      ⁡              [        k        ]            while when the UE is subject to a frequency offset with respect to the base station, the received signal is then
      r    ⁡          [      k      ]        =                    (                  1          +          j                )            ⁢              (                                            h              1                        ⁡                          [              k              ]                                +                                                    (                                  -                  1                                )                                            ⌊                                                      k                    +                    1                                    2                                ⌋                                      ⁢                                          h                2                            ⁡                              [                k                ]                                                    )            ⁢              exp        ⁡                  (                                    j              ⁢                                                          ⁢              2              ⁢                                                          ⁢              π              ⁢                                                          ⁢              k              ⁢                                                          ⁢              Δ              ⁢                                                          ⁢              fT                        +                          φ              0                                )                      +          n      ⁡              [        k        ]            Where hm[k] corresponds to the channel response from antenna m (m=1, 2) and at the time instant of transmission of the kth pilot, Δf is the frequency offset, T is the time duration between two consecutive samples and φ0 is an initial phase rotation.
Some techniques are already known for computing an estimation of the frequency offset based on the computation of phase discriminator.
However, the techniques which are known do not achieve both accuracy in the estimation of the offset and still operate in a relatively wide range of frequency offsets.
It is particularly desirable to be able to estimate an offset in the maximum range allowed by an estimation based on the CPICH, while providing accuracy in the estimation. This maximum range is given by
                          Δ        ⁢                                  ⁢        f                    ≤          1              2        ⁢                                  ⁢        T              ,where T is the time between two consecutive CPICH pilots. In UMTS
  T  =            1      15        ⁢          10              -        3            seconds, which corresponds to covering a range of frequency offsets from minus to plus 7500 Hz.
Such is the goal of the present invention.